Filtration treatment of water for purification by present methods is a troublesome operation. Water is usually fed onto the upper surface of a relatively shallow sand bed, and filtered water is withdrawn from the bottom of the sand bed. The upper portion of the sand bed becomes plugged with the solid materials removed from the water and with algae and other organic materials. The bacterial activity in the sand bed can be only partially controlled, if at all. The filtration rate drops off because of the plugging and contamination of the bed, and the bed must be cleaned and reconditioned, usually by removal of the upper surface of the sand and replacement with fresh sand. Backwashing in the required upward direction can be done only in a very limited manner because of the large amount of backwash water which would be required in view of the large horizontal areas of the beds to obtain a high upward backwash water flow rate.